The present invention relates to a copier, facsimile apparatus, printer or similar image forming apparatus and, more particularly, to an image forming apparatus of the type forming a toner image on a photoconductive drum or similar image carrier, transferring it to a paper or similar transfer medium via an intermediate transfer body, e.g., belt, and fixing it on the medium by a fixing device.
Image forming apparatuses of the type described include a color image forming apparatus which sequentially forms toner images of different colors on an image carrier, sequentially transfers them to an intermediate transfer body one above the other, and transfers the resulting multicolor image from the transfer body to a transfer medium at a time. Also known in the art is an image forming apparatus which transfers a toner image from an image carrier to a transfer medium and fixes it on the medium by a fixing device. This kind of apparatus may be provided with a switching means for switching the speed at which the transfer medium moves through the fixing device, depending on the kind of the medium, e.g., whether or not the medium is a relatively thick paper, e.g., OHP (Over Head Projector) sheet which needs a substantial fixing time. Usually, such switching means is actuated by a signal from a mode switch which is operated by an operator.
Assume that the apparatus of the kind using an intermediate transfer body is constructed to switch even the peripheral speeds of the image carrier and intermediate transfer body when it sets up a lower fixing speed. This brings about a problem that process conditions for the formation of a toner image become complicated and/or the image forming speed is noticeably lowered. For example, in an electrophotographic image forming apparatus, it is necessary to change the charging condition, exposing condition and other process conditions. As a result, not only optimal process conditions each matching a particular peripheral speed have to be determined at great cost, but also complicated control is necessary over the process conditions of the apparatus. In addition, the image forming speed is critically lowered since all the steps, i.e., the toner image forming step to the fixing step are effected at low speed.